Low resistance, linear (current-voltage relationship) contact to n-type GaAs is traditionally done with a Au/Ge/Ni metallurgy. Although commonly used, this metallurgy has several disadvantages. First, it contains gold, which forms an intermetallic compound with aluminum, a metal useful as an interconnect metal. Second, the contact resistivity of the contact degrades when exposed to temperatures near or above its alloying temperature (about 450.degree. C.), and thus the contact is degraded by any high temperature process step following contact formation. Third, metallic phase and/or alloy segregation occurs upon contact formation, thus limiting the minimum size of the contact.
Low contact resistance (R.sub.c) ohmic contacts to GaAs have been produced with Ni and Ge that do not evidence the above problems, but these processes involve growing Ge epitaxially on GaAs, which is not compatible with present GaAs LSI (large scale integration) processes.
Pd/Ge and Ni/Ge ohmic contacts have been formed on GaAs by sputtering and then sintering, but the contact resistance values of contacts using these metallurgies are about 10 to 100 times that of the best Au/Ge-based contacts, and there is evidence that these contacts experience an increase in contact resistance of about 5 to 10 times for prolonged aging cycles above 450.degree. C.